U. S. GEOLOGICAL SURvEv 



MONOGRAPH XIX PLATE XXXIII 



Fig. 1. — Biotitic and muscovittc graywacke. 



Fg 2.— B'otite slate. 



Fig. 3,— Biotite slaie. Fig, 4,— Biotite slate. 



THIN SECTIONS SHOWING DEVELOPMENT OF MICA-SLATES. 



